The treatment prescribed by restorative dentistry for a carious lesion of the enamel or dentin is simply to cut out the affected tissue with a drill. The cavity produced must then be filled in order to restore the strength of the tooth, protect the adjacent tissue from further decay and thermal shock, and, especially in the anterior teeth, restore the natural appearance of the tooth. To fill such a hole, the dentist has several methods at his disposal. All of the techniques have certain advantages and disadvantages but none has all of the properties which Black in 1895 outlined for the ideal restorative material (G. V. Black, "An Investigation of the Physical Characters of the Human Teeth in Relation to their Disease, to Practical Dental Operations, Together with the Physical Characters of Filling Materials," Dent. Cosmos., Vol. 37). Since 1915, silicate cements have been the major material for aesthetic restorations (C. C. Volker, "Dental Silicate Cements in Theory and Practice," Dent. Cosmos. Vol. 58, 1098 (1916)). Such a material has three major weaknesses; it has a high solubility and therefore is subject to wash-out which causes its poor long-term stability, it loses its aesthetic properties by discoloration caused by absorption of oral fluids, and it is a highly brittle material which cannot be used on stress bearing surfaces. Efforts to develop a better material led to the introduction of direct filling resins in the late 1940's. Excessive polymerization shrinkage and poor thermal expansion characteristics were the major weaknesses of these materials. Also, the prevalence of recurrent caries produced by oral fluid ingress along marginal gaps between the restoration and the tooth caused by excessive shrinkage in setting contraindicated continued use of such materials (F. A. Peyton, et al, "Restorative Dental Materials," C. V. Mosby, St. Louis, p. 465 (1964)). Painstaking implacement of these materials by slow build-up of the restoration in thin layers did produce excellent restorations. Wear and wash-out were never considered problematic, but the slow procedure took hours and the resultant cost was prohibitive for the patient.
In recent years a new class of materials has been introduced which is regarded as offering more advantages than any of the earlier efforts, namely, ceramic reinforced polymer matrix composites ("Guide to Dental Materials and Devices," 4th Ed., Amer. Dent. Assoc., Chicago (1968)). Such materials were developed largely through the efforts of R. L. Bowen and are now being widely marketed for use as both anterior and posterior restorative materials.
The composites involve a methacrylate based system in which a silica or silicate glass filler is covalently bonded to a resin matrix, or to a coupling agent which is covalently bonded to both. With fillers used to the 70--80 weight percent level, polymerization shrinkage and thermal expansion are greatly reduced in comparison with direct filling resins. The problems with recurrent caries which was observed in connection with the use of the earlier direct filling resins appears non-existent with the use of the dental composites. In terms of stability, solubility, strength, and general performance, the ceramic reinforced polymer matrix composites overcome many of the objections raised with regard to the earlier silicate cements and direct filling resins.
Despite their advantages, such composites have not been entirely satisfactory from the standpoint of resistance to abrasion and wear, ease of finishing and ease of implacement. The greater hardness of the glass or mineral particles (all of which range from 5.5 to 7.0) relative to the hardness of tooth enamel would be expected to result in restorations of high durability and retentiveness but, contrary to such expectations, and as set forth hereinafter, applicants have now found that such hardness is actually a disadvantage rather than an advantage in several respects.
Other patents and literature showing the state of the prior art are Bowen U.S. Pat. Nos. 3,066,112, 3,200,142 and 3,635,889, Chang U.S. Pat. No. 3,452,437, and the references cited therein; Bowen, R. L. et al, "Composite Restorative Materials," paper given at 50th Anniversary Symposium on Dental Material Research, NBS, Oct. 6-8, 1969; L. J. Brontman et al, "Modern Composite Materials," Addison-Wesley Publishing Co., Reading, Mass. (1967); R. L. Bowen, "Effect of Particle Shape and Size Distribution in a Reinforced Polymer," J. ADA, Vol. 64, p. 481 (1964); W. Souder et al, "Physical Properties of Dental Materials," National Bureau of Standards Circular C433, U.S. Government Printing Office, Washington, D.C. (1942).